This invention pertains to an insulative cabinet wall comprising a barrier film used to protect plastic resins employed in the construction of the wall from attack by solvents. More specifically this invention relates to the use of barrier films consisting essentially of a linear polyethylene polymer to protect styrenic resins such as, for example, high impact polystyrene (HIPS) or acrylonitrile-butadiene-styrene (ABS) copolymers.
Styrenic resins are frequently susceptible to solvent attack by compounds, especially the halocarbon compounds used as blowing agents in the preparation of the insulative material contained within, for example, an appliance unit such as a refrigeration or boiler.
The styrenic resin may be attacked by the halocarbon compounds becoming weakened and susceptible to failure and fracture. Material susceptible to attack in such manner is not desirable in an appliance unit as it can lead to loss in the overall thermal insulating efficiency of the unit and in some instances give rise to structural strength problems and eventual deformation of the unit.
It is known that the composition of the styrenic resin may be modified to increase its resistance to attack from solvents. Greater resistance to some solvents, especially halogenated solvents, can be obtained by increasing the acrylonitrile content of an ABS copolymer or introducing a greater rubber content, see for example, U.S. Pat. No. 4,144,206. However, such a solution is not always feasible as other physical properties of the material such as impact strength or more critically moldability may change making them unsuitable for the intended application
An alternative to modifying the composition of the styrenic resin is to protect such material by the use of a barrier film or layer, wherein the insulative material is prevented from contacting the styrenic resin see, for example, U.S. Pat. No. 3,960,631. In this document the use of a coextruded film comprising a low density polyethylene and an ethylene acrylic acid copolymer is disclosed. The film provides a physical barrier preventing adhesion of the insulative polyurethane foam to the styrenic resin. Preventing such adhesion greatly reduces environmental stress crack failure thereby significantly reducing the formation of fracture sites in the styrenic resin. Such fracture sites are the principal points where solvent attack can subsequently take place weakening and eventually allowing the polymer matrix to break. Similar barrier films serving the purpose to restrict adhesion and minimize stress crack failure are also disclosed in U.S. Pat. No. 4,707,401 and U.S. Pat. No. 4,196,950. In U.S. Pat. No. 4,005,919 use of barrier films in conjunction with ABS liner material is disclosed wherein the barrier film is a rubber-modified high nitrile polymer.
Such barrier films as described above are capable of offering some protection indirectly by minimizing occurrence of stress failure sites of the liner from attack by the halogenated compounds such as, for example, trichlorofluoromethane (Refrigerant-11) frequently employed in the manufacture of polyurethane foam. However, the continued use of certain fully halogenated compounds and especially trichlorofluoromethane is undesirable in view of the current opinion that their presence in earth's upper atmosphere may be a contributory factor in the recently observed reduction of ozone concentrations.
Recent developments in polyurethane technology has led to the identification of certain hydrogen-containing halogenated carbon compounds as being suitable physical blowing agent replacements for the traditionally employed fully halogenated chlorofluorocarbon compounds. Such recently identified alternative blowing agents are the "soft" chlorofluorocarbon compounds (HCFCs) and include dichlorotrifluoroethane (Refrigerant-123) and dichlorofluoroethane (Refrigerant-141b). These compounds are described as "soft" compounds due to the presence of hydrogen on the carbon backbone and are characterized by having very low or negligible ozone depletion potentials in contrast to the "hard", fully halogenated compounds.
However, in recent evaluation studies of the "soft" chlorofluorocarbon compounds a severe problem of attack on the styrenic resins used as liner material in the preparation of insulative cabinet walls has been observed. Liners comprising a film present for the purpose of preventing adhesion of polyurethane foam to the styrenic resin, such as described in U.S. Pat. No. 3,960,631, are also apparently susceptible to attack by such "soft" chlorofluorocarbon compounds. Thus indicating, that the material currently used as a physical adhesion barrier does not have sufficient chemical barrier properties to prevent attack by the halogenated blowing agent.
It is also apparent from evaluation studies that the aggressivity of some of the HCFC compounds, particularly Refrigerant-123 and isomers thereof, towards styrenic resins typically used in the construction of insulative cabinet walls is too great to be conveniently overcome by chemical modification of the composition of the styrenic resin without, for example, a significant loss in the desirable moldability of the resin.
It is therefore of interest to consider the possibility of modifying or using an alternative barrier film. Desirably, such an alternative barrier film should allow for efficient preparation of the shaped liner employed in the appliance unit and more importantly minimize or prevent attack of the styrenic resin by blowing agents, especially the newly identified HCFCs, used in manufacturing the polyurethane foam.